Hi I have an old AE-1 that appears to mostly work. The problem is that the shutter speed control does not change the shutter timing. When set to B, 2s, 1s etc it is easily noticeable that the setting dial is making no difference. In fact, if you open the back and release the shutter, the curtains don't even appear to open. Anyone know if this is a common problem and easy to fix oneself? Ta.